NAME

SYNOPSIS

DESCRIPTION

In a System V-like environment, one has the two types mcontext_t and
ucontext_t defined in <ucontext.h> and the four functions getcontext(),
setcontext(), makecontext(3) and swapcontext(3) that allow user-level
context switching between multiple threads of control within a process.
The mcontext_t type is machine-dependent and opaque. The ucontext_t
type is a structure that has at least the following fields:
typedef struct ucontext {
struct ucontext *uc_link;
sigset_t uc_sigmask;
stack_t uc_stack;
mcontext_t uc_mcontext;
...
} ucontext_t;
with sigset_t and stack_t defined in <signal.h>. Here uc_link points
to the context that will be resumed when the current context terminates
(in case the current context was created using makecontext(3)),
uc_sigmask is the set of signals blocked in this context (see
sigprocmask(2)), uc_stack is the stack used by this context (see
sigaltstack(2)), and uc_mcontext is the machine-specific representation
of the saved context, that includes the calling thread’s machine
registers.
The function getcontext() initializes the structure pointed at by ucp
to the currently active context.
The function setcontext() restores the user context pointed at by ucp.
A successful call does not return. The context should have been
obtained by a call of getcontext(), or makecontext(3), or passed as
third argument to a signal handler.
If the context was obtained by a call of getcontext(), program
execution continues as if this call just returned.
If the context was obtained by a call of makecontext(3), program
execution continues by a call to the function func specified as the
second argument of that call to makecontext(3). When the function func
returns, we continue with the uc_link member of the structure ucp
specified as the first argument of that call to makecontext(3). When
this member is NULL, the thread exits.
If the context was obtained by a call to a signal handler, then old
standard text says that "program execution continues with the program
instruction following the instruction interrupted by the signal".
However, this sentence was removed in SUSv2, and the present verdict is
"the result is unspecified".

RETURNVALUE

When successful, getcontext() returns 0 and setcontext() does not
return. On error, both return -1 and set errno appropriately.

ERRORS

None defined.

CONFORMINGTO

SUSv2, POSIX.1-2001. POSIX.1-2008 removes the specification of
getcontext(), citing portability issues, and recommending that
applications be rewritten to use POSIX threads instead.

NOTES

The earliest incarnation of this mechanism was the setjmp(3)/longjmp(3)
mechanism. Since that does not define the handling of the signal
context, the next stage was the sigsetjmp(3)/siglongjmp(3) pair. The
present mechanism gives much more control. On the other hand, there is
no easy way to detect whether a return from getcontext() is from the
first call, or via a setcontext() call. The user has to invent her own
bookkeeping device, and a register variable won’t do since registers
are restored.
When a signal occurs, the current user context is saved and a new
context is created by the kernel for the signal handler. Do not leave
the handler using longjmp(3): it is undefined what would happen with
contexts. Use siglongjmp(3) or setcontext() instead.